How to position a poncho with cloth physics
I'd check out this answer from Faceb Faceb. My first thought is to grab a few vertices on your poncho, and assign them to a group. In your cloth physics settings, check the "pinning" option. You should be able to use that same vertex group to parent to the cloth to the shoulder of your character. The tricky part is, you'll have to 'pose' the cloth in edit mode all stretched out, and when you apply physics, the cloth should fall into place, and the pinned verts should stick to your object. Check out more here: 

Pinning cloth to moving model